1. Field of the Invention
This invention relates to X or Gamma ray indirect imaging detectors and more specifically to wirebond protection without use of an encapsulant.
2. Description of the Related Art
X or Gamma ray indirect imaging detectors include a conversion layer or “scintillator” that converts X or Gamma rays to visible light, an optional fiber optic plate (FOP) that transfers light to an imaging plane and a visible band imaging sensor at the imaging plane. For X rays, phosphor or cesium-iodide are typical conversion layer materials. The FOP protects the imaging detector from being damaged by the high energy X or Gamma rays, prevents the sensor from reading those rays as light and takes the image created by the conversion layer and focuses it on the imaging sensor keeping the image sharp and coherent. Indirect imaging leverages the established visible band CCD and CMOS imaging technologies.
To read out the image, the imaging sensor is mounted onto a printed circuit board (PCB). Electrical contact pads in a non-imaging region on the surface of the imaging sensor are wirebonded to electrical contact pads on the PCB, which are attached to read out and other processing circuitry. The wire bonds are very fragile and susceptible to damage or failure due to corrosion or mechanical stress that may occur during final assembly and handling. The failure of even a single wire bond can result in the loss of the detector.
To protect the wire bonds, a “glob top” of material is used to encapsulate the wire bonds. A typical material is a low aspect ratio (AR) clear acrylate that will readily flow around the wire bonds to encapsulate them. The material is typically cured using UV light. This is industry standard approach to protect wire bonds in many different semiconductor packages.
More recently, X-ray detectors are being designed in “multi-tile” configurations in which each tile includes one or more imaging sensor dies to provide a larger and higher-resolution detector. The detector may be >30 mm per side. The die typically have a “surface flatness” that is far superior to that of the PCB. To address the surface flatness, on some architectures the imaging sensor and PCB are mounted side-by-side on a tile carrier having a surface flatness comparable to the die. The imaging sensor is wire bonded to the PCB and a glob top is used to encapsulate the wire bonds. See FIG. 5-2 of Jan Bosiers et. al., “An overview of Teledyne DALSA Professional Imaging CCD and CMOS capabilities” Proceedings 2013 Scientific Detector Workshop, Firenze, Italy, October 2013.